PHNOM PENH, Cambodia − Scientists in 17 countries will scout waterways to locate and study the world's largest freshwater fish species, many of which are declining in numbers, hoping to learn how to better protect them, researchers announced Thursday.

Scientists will look for creatures such as the Mekong giant catfish, goliath catfish, giant stingrays, razor-toothed gars, massive carps, caviar-producing sturgeon and predatory salmon -- which can all weigh more than 200 pounds and grow to six feet or longer, the World Wildlife Fund said in a statement received Thursday.

The animals are "unique" and "disappearing fast," said Zeb Hogan, who will lead the project funded by WWF and the National Geographic Society.

Hogan, who researches the Mekong giant catfish in Cambodia, said scientists will tag fish to track their movements in hopes of better understanding their migration paths and why they're dying off.

"These fish are the proverbial 'canaries in the coal mine' for freshwater ecosystems," the statement quoted Hogan saying. "Their disappearance is often the first warning sign of overfishing or other trouble in the rivers and lakes where they live."

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Some of the freshwater giants are listed on the World Conservation Union's Red List of Threatened Species.

The Mekong giant catfish -- which shares the title of largest freshwater fish with a close relative, the dog-eating catfish -- was listed as critically endangered last year after research showed its numbers had fallen by at least 80 percent over the past 13 years.

"These giants are the freshwater equivalents of elephants and rhinos, and if they were visible to us on land the world wouldn't stand by while they disappeared," Robin Abell, a WWF biologist, was quoted saying. "In the end, we'll know better how to manage fishing and protect habitats to save the species for the future."

Officials from the two sponsoring agencies could not be reached Thursday for more details, including the countries where the research would take place.

The project was launched to coincide with the start of a World Conservation Union meeting in Bangkok, Thailand, of more than 6,000 government officials, scientists, executives and environmentalists.

The meeting is held every four years to outline new threats to endangered plants and animals. The union's membership includes 81 states, 114 government agencies, more than 800 private groups and 10,000 scientists from 181 countries.